A number of framing designs have already been proposed for the assembly of panels. Five of the most applicable designs are reviewed below.
U.S. Pat. No. 3,380,210 is intended for aluminum. To achieve sufficient deformation of the locking mechanism out of this material, the design uses two hooked tongs which lock onto toothed surfaces. The arrangement of the tongs is such that when the beam is used in a horizontal mode, any deformation of the beam increases the pressure on the tongs. This can make the cover plate more difficult to install and remove in this mode. In addition, this design would concentrate bending stress along a fairly narrow cross-section of the tongs, making the design prone to failure at their base. Furthermore, the fairly complex and thick cross-section of the cover plate makes it impossible to overlap the cover plates at beam junctions without first machining away some of the material.
U.S. Pat. No. 3,932,974 is a four component design which includes a toothed rubber T-shaped gasket which fits into a toothed metal slot. The rubber, being fairly soft, requires a fairly coarse tooth size. This make it difficult to achieve a snug fit with the panel to be held. To compensate for this the gasket does not fit into the beam directly. Instead, it fits into a third component, a metal rail which then fits into the main beam. The tension required to hold the panel against the rail is achieved by screws which, when tightened, draw the rail and gasket assembly into the beam. A fourth component, a cover plate, is required to hide the screw mechanism from view. The four part design is complex and its installation is expected to be fairly labour intensive.
U.S. Pat. No. 4,648,231 uses a three piece design: a structural beam, a cross shaped member with two tongs which fit into it, and a cap with another two tongs which fit into the cross shaped member. As noted above, the tonged designs, required for aluminum, have a number of disadvantages including susceptibility to failure and excessive thickness. When used in the horizontal position, downward slippage of the panels, such as could result from normal diurnal temperature changes, could eventually put the entire panel load on the locking teeth.
Canadian Patent #953,070 covers a design for the assembly of partitions. In this design, clips are fixed onto the partitions. A "synthetic resin" core is used to connect the clips together. By its design, the partitions need to be thick enough to hide the clip and resin core assembly, thus making it inapplicable for glass or other thin panels.
Canadian Patent #1,066,472 covers another design for the assembly of panels. The design also utilizes "keys or clips", attached to adjacent panels, as above, or "metallic sections" with a cover plate to hold thinner panels. Being primarily designed for aluminum however, the design is inappropriate for a more elastic material such as PVC or ABS plastic. Because of the lower elasticity of the metal, the design requires two tongs for adequate compliance in order to engage the cover plate adequately. The two tong design requires a greater beam and connector thickness and requires an additional glazing to cover the connector. A more important shortcoming however, is that when the beam is utilized in a horizontal mode to support a panel, the weight of the panel is transferred directly to the toothed extension of the connector. This creates a high bending stress at the point at which the toothed extension joins the main part of the beam, making it prone to failure, as well as making it difficult to replace damaged panels.
All of the prior art appears to concentrate on applications to extruded metal, primarily aluminum. As a result of the inherent strength of the materials, none of the earlier designs included connector blocks or other similar devices to reduce stress concentrations at junctions between beams.